The present invention relates to vehicle wheel alignment systems, and in particular to an improved non-contact vehicle wheel alignment system utilizing one or more optical imaging sensors to acquire images of a vehicle from which three-dimensional information on one or more current vehicle wheel alignment angles can be obtained.
Various systems have been designed to determine vehicle wheel alignment angles. Originally, vehicle wheel alignment angles were measured directly by alignment angle sensors mounted to the vehicle wheels. For example, U.S. Reissue Pat. No. 33,144 to Hunter et al. and U.S. Pat. No. 4,319,838 to Grosssman et al. each describe vehicle wheel alignment systems which use electro-optical transducers to determine the toe alignment angles of a vehicle. In these systems, six or more electro-optical transducers are carried by support assemblies which are mounted to the vehicle wheels, two on each front wheel, and one on each rear wheel. The '144 Hunter et al. patent and the '838 Grossman et al. patent further describe and illustrate the various wheel alignment angles which can be directly measured using the electro-optical transducers, and how to compute additional angles from these directly measured angles.
In addition to the electro-optical transducers used to measure wheel angles, a gravity-referenced inclinometer, such as shown in U.S. Pat. No. 4,879,670 to Colarelli may also be used. Gravity-referenced inclinometers can be mounted to the vehicle wheels to provide measurements of the wheel camber angle, assuming that the vehicle, while being measured, is resting on a flat and level surface. SAE Publication No. 850219, entitled “Steering Geometry and Caster Measurement” by January, derives and discusses the industry standard procedures and methods by which toe and camber wheel alignment angle transducers are used to determine the caster and steering axis inclination (SAI) angles of the vehicle.
Equipment of this general type, and the use of the apparatus and method set forth above, is well known in the vehicle wheel alignment industry. Such equipment is capable of determining the camber, caster, and toe alignment angles of the wheels of a vehicle relative to one or more appropriate reference axes, and is sufficient to permit proper adjustment of the vehicle wheel alignment so as to reduce tire wear and provide for safe vehicle handling.
An alternative type of vehicle wheel alignment system which has become increasingly popular is what is commonly referred to as an “external reference aligner”. U.S. Pat. Nos. 4,745,469 and 4,899,218, both to Waldecker et al. illustrate how lasers may be used to illuminate the tires of a vehicle with structured light, and cameras used to capture images of the illuminated tires. These patents further describe how “machine vision techniques” are employed to process the obtained images and to determine distances between the cameras and locations on the tire sidewalls, thereby allowing a determination of the locations and orientations of the vehicle wheels in a coordinate system relative to the cameras. Subsequent processing of the determined locations and orientations identifies conventional wheel alignment angles such as toe, camber, and caster for the imaged vehicle wheels.
German Patent No. DE 29 48 573 A1, assigned to Siemens AG, describes the use of cameras to determine the locations and orientations of the wheels of a vehicle. On one side of the vehicle, a single camera is moved to multiple positions to view the vehicle wheels and obtain multiple images of the known ellipse formed by the wheel rim. Alternatively, a single fixed camera is used at each side of the vehicle in conjunction with movable mirrors, or multiple cameras may be used to obtain the multiple images. The system examines the sets of images of the rim ellipses thus viewed for each of the wheels of the vehicle, identifies major and minor ellipse axes, and subsequently determines the locations and orientations of the wheels. Using this determined information, the conventional wheel alignment parameters are identified.
Continued development in the industry of “external reference aligners” is shown in a series of patents related to U.S. Pat. No. 5,675,515 to January, and in a series of patents related to U.S. Pat. No. 5,535,522 to Jackson. These patents describe and disclose various embodiments of “external reference aligners” and vehicle wheel alignment systems in which optical targets of known configurations having contrasting surfaces are mounted to the wheels of a vehicle undergoing alignment. Images of the known optical targets are obtained by one or more camera systems, and processed to determined relative positions, orientations, and rotational axes in three-dimensional space. From the relative positions, orientations, and rotational axes of the targets, corresponding information about the associated vehicle wheels is calculated, and conventional wheel alignment angles identified.
The optical targets of known configurations employed in conventional “external reference aligner” systems typically consist of high contrast geometric shapes such as circles, squares, or triangles. The accuracy of such an optical target is dependent upon how well the high contrast edge of the geometric shapes can be located in the pixellated image produced by the machine vision camera system observing the target. For the best accuracy, the high contrast geometric shapes must be separated from each other on the face of the target by a distance which is sufficient to prevent the apparent fusing of adjacent shapes into a single shape when reduced edge sharpness causes two shapes to bleed into the same image pixel. These factors combine to limit the number of pixels in an image whose values are utilized to calculate the position and orientation of the optical target to a very small portion of the pixels in the image which represent the target. Typically, only a small percentage of the pixels in the image of a target are used to calculate the position and orientation of a high-contrast optical target.
Accordingly, an increase in performance and functionality of external reference vehicle wheel alignment systems would be greatly beneficial. One possibility for increasing performance and functionality would be to provide a target design which provides a high percentage of useful image pixels for purposes of determining target position and orientation. An alternative improvement would be the elimination of the need for costly known optical targets in a system which is capable of determining the position, orientation, and rotational axes of one or more vehicle wheels directly from images obtained by one or more imaging systems configured to observe and track multiple points on a vehicle wheel without the need for highly precise attached optical targets.